The reservoirs are cleaned regularly (although clearly this does not include the "bathtub ring" around Reservoir 6 - ahem).
The Water Bureau sends public service notices out via email. They frequently send these updates to NextDoor and to our Mt Tabor Neighborhood Association (MTNA) board members (who forward the messages to our MTNA listserve).
Link to our 1/15/17 "Mt Tabor Neighborhood Social Media and Email Groups" blog post for information about signing up for those services.
Portland Water Bureau reservoir cleaning and other Mt Tabor Park news updates are also posted at the Friends of Mt Tabor Park website (but please note that the "details here" link sometimes takes you to a Facebook link that requires you to log or sign in).